1. Field of the Invention
The present invention relates to an imaging optical system suitable for an image forming apparatus such as a copier and a printer that employ a semiconductor laser array having a plurality of independently controllable laser elements, and an image forming apparatus using the imaging optical system, and more particularly to an imaging optical system and an image forming apparatus that provide a small-size design and high resolution.
2. Description of the Prior Art
In an image forming apparatus such as a laser printer and a copier that scan a laser beam, a method of scanning by a polygon mirror is commonly used. However, image forming apparatuses employing the polygon mirror have a problem in that they cause mechanical vibration and noise because of the mechanical rotation of the mirror and a high scanning speed is difficult to achieve because of the low reliability of a motor for rotating the mirror and the limitation of rotation speed. Also, different scanning line densities, bent scanning lines, different scanning densities, or the like arise because of the surface inclination of each mirror, and the precision of manufacturing f.theta. lens, causing degradation of image quality. Accordingly, there is a demand for a method of scanning a laser beam without using such a polygon mirror.
A conventional image forming apparatus employing a method of scanning without using a polygon mirror is disclosed in Japanese Published Unexamined Patent Application No. Hei 10-16297, for example.
FIG. 15 shows the image forming apparatus. The image forming apparatus comprises a semiconductor laser array 1 as a light source having a plurality of laser elements 1a arranged linearly, and an imaging optical system 4 as an imaging optical system of a magnification optical system for imaging laser beams 2 from the laser elements 1a on a photosensitive surface 3a. The imaging optical system 4, which constitutes a telecentric optical system between the composite focus of the photosensitive surface 3a side and the light source, comprises a group of imaging lenses. By this construction, printing can be performed over a short side (an image width of 210 mm) A4 size at a rate of 200 dots per inch.
However, the conventional image forming apparatus has a problem in that, since the incidence surface of the telecentric optical system, located nearest to the light source, is distant from the light source, a laser beam incident on the telecentric optical system is spread, with the result that an effective diameter of lens must be increased, resulting in lenses being enlarged.
Also, there is a problem in that the short distance between the composite focus of the photosensitive surface side and the incidence surface of the telecentric optical system increases the bend of a principal light beam of the laser beam 2, with the result that high resolution cannot be obtained because of an increased aberration. Therefore, recently demanded resolutions of 600 dots or more per inch for A3 size cannot be achieved.